LETTER OF THE GEOLOGIST. XVII 



tude 108° 15' ; then descended to tlie Grand and followed it up to longi- 

 tude 1070 35', and thence via the White Eiver (Ute) Indian agency to 

 Eawlins, where they arrived on October 23. 



The whole area worked is about 3,500 square miles, in surveying which 

 about sixty stations were made. 



The geological work of this division, by Dr. Peale, connects directly 

 with that done by him in 1874 and 1875. Sedimentary formations pre- 

 vail in both districts visited during the past season. 



The country first examined lies between the San Miguel and Dolores 

 Eivers, extending northward and northwestward from Lone Cone 

 Mountain. The general character of this region is that of a plateau cut 

 by deep gorges or caGous, some of which, especially toward the north, 

 extend from the sandstones of the Dakota group to the top of the Bed 

 Beds. The depth of the caiion, however, is no indication of its impor- 

 tance as a stream-bed, for, excepting the main streams, it is dry the 

 greater portion of the year. There are not great disturbances of the 

 strata, what folds do occur being broad and comparatively gentle. 



The San Miguel Eiver, on leaving the San Juan Mountains, flows 

 toward the northwest, and, with its tributaries, cuts through the sand- 

 stones of the Dakota group, exposing the variegated bods lying beneath, 

 that have generally been referred to the Jurassic. About 25 or 30 miles 

 north of Lone Cone, the river turns abruptly to the west and flows west 

 and southwest for about 15 miles, when it again turns and flows gener- 

 ally northwest, until it joins the Dolores. Between the San Miguel and 

 Lone Cone the sandstones of the Dakota group, or No. 1 Cretaceous, 

 are nearly horizontal, forming a plateau which, on approaching the 

 mountains, has a cropping of Cretaceous shales. 



Beyond the bend, the San Miguel flows in a monoclinal valley, in 

 which the caiion walls are of the same description as in the upper part 

 of its course. As the mouth is approached, the Eed Beds appear. 

 Between this portion of the course of the San Miguel and the almost 

 parallel course of the Dolores, which is in a similar monoclinal rift, there 

 are two anticlinal and two synclinal valleys parallel to each other. 

 They are all occupied by branches of the Dolores. Lower Cretaceous, 

 Jurassic, and Triassic strata outcrop, and present some interesting geo- 

 logical details, which will be fully considered in the report on the district. 

 The Dolores Eiver comes from a high plateau in a zigzag course, flowing 

 sometimes with the strike, and sometimes with the dip of the strata. 

 Its general course on the western line is about northwest, from which it 

 turns to the northward and westward, finally changing to northwest 

 again, to its junction with the Grand. It is in canon the greater part of 

 its course. 



In the region of country north of Grand Eiver, the geological forma- 

 tions extend uninterruptedly from the Eed Beds exposed on Grand 

 Eiver to the white Tertiary cliffs forming the summit of the "Eoan 

 Mountains," or Book Cliffs. The Grand is generally in a caiion in the 

 II a 



